1. Field of the Invention
This invention pertains to cathode-ray tube deflection systems, and more particularly to precise deflection convergene systems for shadow-mask multi-gun mixed-color cathode ray tubes.
2. Summary of the Prior Art
Color television being the commercial reason for making color kinescopes, the conventional tube and its associated circuits economically perform adequately for use in the 525-line raster, 60-hertz field, 30-hertz frame, interlaced system standard in the jurisdiction of the U.S. Federal Communications Commission. Dots of screen materials cathodo luminescent in three different colors are applied to the tube face in a regular pattern; three differently glowing dots always have the same relative position and each such dot is protected from the edge of an incident beam directed to another spot by a perforated shadow mask which permits a beam to bombard with its high-density core the luminescent spot at which it is aimed, while its low density edge is caught by the mask. With such a structure it is preferred for simplicity to use three separate electron guns, one for each color, in order that variously proportioned mixtures of two or all three of the different colors available from the three kinds of dots may be provided by appropriate modulation of the beam from each separate gun, rather than by the complex feat of time distribution, over the different kinds of dots, of the beam from a single gun. The three separate guns are located symmetrically around the central axis of the tube, each with the gun axis coplanar with the central axis of the tube. Conventionally, each gun is provided with a focussing permanent magnet, known as a "color purity" magnet, to collimate its electron beam as nearly perfectly as may be, and each of the three beams is then caused to pass through the field of a deflecting magnet, known as a static convergence (or, for brevity, convergence) magnet, which is so adjusted that its associated electron beam will pass through a deflection volume common to all three beams, and then, in the absence of any deflecting field in the deflection volume, will converge to a "point" on the screen to which the other two beams also converge. This "point" is in fact an area unit containing three spots, each luminescent in a different color; and the convergence magnet for each gun is adjusted to cause its beam to impinge through the proper shadow mask aperture upon a spot whose luminescence color is associated with that gun. From the observer's viewpoint the "point" area unit appears, at normal viewing distance, to be truly a point, since the spacing of the three spots is necessarily less than the resolution capability of the normal human eye in order that mixtures of light from two or three differently colored spots will appear as a single visible spot of a single color produced by the blending. When a deflection yoke, conventionally external to the tube envelope around the deflection volume, produces in the volume a magnetic deflecting field substantially uniform over the volume, the three electron beams will all be deflected, to a first approximation, through the same angle, and will move appropriately to the same three-dot "point" or area unit, each beam arriving at its appropriate phosphor dot.
Practical considerations impair this first approximation. First, because the beams from the different guns enter the deflection volume at an angle with the tube axis, each will be deflected slightly differently, and they will not converge all over a screen even if it is spherical with its center in the deflection volume, so that all parts of the screen are at the same distance from the place where deflection occurs. Secondly, the radius of the spherical segment which is the tube face is conventionally greater than the distance from the virtual center in the deflection volume where the axes of the three beams, extended backward from their convergence point would intersect. Consequently, if the beams are deflected off the tube central axis, around which the three electron guns are disposed, they will actually converge before reaching the screen, and will diverge beyond that convergence point, where they reach the screen. To compensate for this it is necessary to add to the constant field from each convergence magnet a field which is a function of the deflection caused by the field of the deflection yoke. This may be done by a winding on the structure of the convergence magnet, whose constant field may be provided by a continuous current either as a component of the total current in the same winding, or in a separate winding. The prior art discloses the use of separate windings to receive separately currents corrective of the effects of vertical and of horizontal deflection. Since windings on magnetic structures are necessarily inductive, producing lags between an applied voltage and the current it produces, ingenious circuitry is, in general, applied to produce adequately close approximation to ideal current wave forms with economically feasible applied voltages. The problem is simpler for fixed horizontal and vertical deflection frequencies, such as occur in television receivers, then for instruments in which these frequencies may be required to be varied.
Television Engineering Handbook, ed. D. G. Fink, McGraw-Hill Book Company, New York, 1957, pp 6-75 through 6-80, and chapter 6 in general, discloses that in a simplified case the required dynamic convergence fluxes (and hence currents) are proportional to squares plus linear functions of the vertical and horizontal deflection amplitudes. Welsh and McCormick, U.S. Pat. No. 3,557,031, column 9, line 45 through colume 10, line 44, similarly describe the required functions for convergence as composed of a sum of a square and a linear function of the vertical deflection and a sum of a square and a linear function of the horizontal deflection. The square functions required are produced by full-wave rectification of the respective deflection voltages, following by a non-linear circuit to give the required square characteristic. Spannhake, U.S. Pat. No. 3,613,108, teaches the addition, to convergence currents of the square plus linear nature, certain inputs from a pincushion distortion correction circuit provided for improving the deflection circuitry to make a more nearly rectangular raster for television. Rhee, U.S. Pat. No. 3,708,715, teaches particular means to combine parabolic and linear wave forms.
None of the published prior art known to me discloses the provision of a product of the vertical and horizontal deflection amplitudes as a component of the convergence correction function, nor the use of cubic functions.